The invention relates to a lighting device for installation in a substantially planar surface.
Lighting devices of the aforementioned type are particularly useful in airports, and especially for use as marking of the correct path for incoming aircraft or aircraft taxiing on the runway, or to make centre lines or limits visible to the pilots.
A number of lighting devices for installation in a plane surface, and in particular for use in airports, are previously known. A device of the type mentioned above is taught in U.S. Pat. No. 4,860,182. This device comprises a prism holder for installation in a plane, preferably horizontal surface, and in particular in a runway. The prism holder has two orifices in its upper surface. A light refracting means in the form of a prism grating is arranged in each orifice. Each prism grating consists of parallel glass discs having a common longitudinal direction and of uniform thickness that are bonded adhesively to one another. The prism gratings preferably consist of borosilicate glass, and a metal foil may be placed between each disc. Each prism grating is secured in place in the prism holder by means of an adhesive.
Positioned in the lower part of the prism holder are two angle-set light sources, each having a reflector arranged to emit light in rays that are essentially parallel in a principal direction. Each light source and reflector is inclined in such a manner that the principal direction of the light source is parallel to the common longitudinal direction of the parallel discs in the associated prism grating. The inclination, expressed as the angle that the principal direction of the light sources forms with the exit face, is said to be 45xc2x0.
The end of the prism grating that faces the corresponding light source forms an incident face that is essentially at right angles to the principal direction of the light sources. The incident face may be plane, convex or concave. In the first-mentioned case, the parallel light beams from a light source fall essentially perpendicular to the plane incident face or plane of incidence. The prism grating also has a plane exit face, which is parallel to the surface in which the device is to be located. The light in the prism grating strikes the plane of reflection at an angle that is not a right angle. Therefore, the light will be refracted so that it is transmitted at an angle of reflection determined by the refractive properties of the prism grating and the aforementioned angle of 45xc2x0.
When installed and used in a runway, the prism holder is inserted into a recess in the runway, with a resilient layer between the bottom of the recess and the prism holder. The top face of the prism holder has projections on either side of each of the prism grating orifices. The height of the projections corresponds to the thickness of the resilient layer. If a sufficiently large force is exerted on the top face of the prism holder, for example, by an aircraft passing with one wheel directly on the prism holder, the layer will yield elastically so that the prism holder temporarily moves slightly downwards in the recess.
A first disadvantage of this known device is that the projections in use form irregularities or bumps for passing aircraft and other vehicles. In particular, the projections are an obstacle for snow clearing in the winter season. Moreover, a sand-blast effect could easily be produced by jet engines blowing sand and gravel from the ground towards the prisms. The projections around the prisms ensure that sand and gravel remain near the prism and swirl around in the vicinity thereof for varying periods of time. In the course of this time, the sand makes dents and scratches in the prism surface. This effect will be particularly noticeable when the prism is at an angle relative to the ground. Moreover, dirt and snow or slush will accumulate around the prism when the ground is cleared or swept. The brushes of sweeping equipment themselves will strike against the prism and might damage it. Therefore, after a certain period of use, there will be a great reduction in the light intensity. The requirements set by the International Civil Aviation Organization (ICAO) relate to new lighting equipment and there are no requirements as regards the properties of the lighting equipment after a period of use. For this reason, there is a great deal of equipment that when new will satisfy the ICAO""s requirements, but which deteriorates rapidly.
A second disadvantage of the known device consists of the weaknesses associated with the elastic flexibility of the prism holder in the vertical direction. This flexibility causes operational problems over time, due in part to the fact that the properties of resilient layer between the bottom of the recess and the prism holder may alter after a certain period of use. The properties are affected by temperature variations, water/moisture and other environmental factors. Furthermore, the flexibility can be inhibited over time, or an undesired movement in other directions may develop because of the ingress of foreign elements, or because of structural changes in the recess in the surface in which the device is installed.
The first and second disadvantages are both related to the fact that each prism grating in the known device withstands only a highly limited impact force from above. This is due to the fact that the prism grating is secured in place in the prism holder by means of an adhesive without any appreciable support against a lower supporting structure. Thus, one of the purposes of the elastic movability in the vertical direction is to prevent damage to or loosening of the prism gratings.
A third disadvantage of the known device is that there are difficulties in gaining access to the internal components, for example, when changing lamps or carrying out other maintenance operations.
A fourth disadvantage of the known device is that there are difficulties in removing or replacing the prism gratings as they are permanently bonded to the prism holder by means of adhesive.
A fifth disadvantage of the known device is that the light sources are not utilized in a sufficiently effective manner, as each light source is placed at a relatively great distance from the end of the associated prism grating facing the light source.
A sixth disadvantage of the known device is that the disclosed angle of 45xc2x0 between the principal direction of the light source and a vertical axis does not give an optimum angle of reflection to ensure desired visibility from certain distances from the device and heights above the plane in which the device is to be installed.
A seventh disadvantage of the known device is that the prism holder and the prism gratings form a relatively large hollow space, where temperature gradients and temperature changes can cause condensation.
An eighth disadvantage of the known device is that the useful life of the light sources is shorter than desired, which is due in part to the fact that the heat generated by the light sources is not conducted away in a satisfactory manner, and causes a higher operating temperature than the optimum.
The lighting device described in Norwegian Patent Application 19996408, and from which the present application claims priority, solves most of the aforementioned problems in a satisfactory manner. In the application, the stresses on the prism are considerably reduced. The sandblast effect and the stresses from snow-clearing and sweeping equipment are reduced to a minimum, as the prisms and the surrounding structure lie in one and the same plane, namely the ground plane. Sand and sweeping equipment will therefore meet a plane surface that sand and brushes can sweep over without causing any significant damage. There will be no reason for snow, ice and dirt to accumulate around the prism.
However, it has been found that there are still some problems for which the last-mentioned patent application does not describe a satisfactory solution. Firstly, it has been found that with the lighting device, as defined in NO 19996408, it has not been fully possible to meet the stringent requirements that the International Civil Aviation Organization (ICAO) has set for airport lighting. To meet these requirements as regards lighting on taxiways involving lights having so-called narrow scattering, it is imperative, inter alia, that the average value of the light intensity within a horizontal angle range of +/xe2x88x9210xc2x0 and a vertical angle range of from 10 to 80 from the ground should not be less than 200 cd (Candela (cd) is a light intensity measurement that takes into account the distance from the light source). It has not been possible to satisfy this requirement fully with the light source according to NO 19996408. This is because the light intensity is too weak close to the ground. However, the horizontal angle range is covered well, as is the vertical angle range of from 3xc2x0 above the ground to some 10xc2x0 above the ground.
In the light of these deficiencies, attempts have been made to reduce the angle xcex2 with which the light beam strikes the exit face of the prism, i.e., the angle between the exit face and the longitudinal axis of the prism, from about 48xc2x0 to about 46xc2x0. However, this reduced angle xcex1xcex2 resulted in such a substantial weakening of the light intensity that the requirements were satisfied to an even lesser degree.
According to the present application a solution has now been found where the requirements as regards average light intensity within the given angle ranges are satisfied. This is accomplished according to the invention by supplementing the angle xcex2 with a certain angle of inclination on the exit face of the prism to such a degree that the combination of angle xcex2 and the angle of inclination lowers the light cone sufficiently to meet the light intensity requirements close to the ground.
Furthermore, tests have shown that the edges of the prism around the exit face are highly susceptible to damage. This is particularly true of the front edge where the exit face meets the longest side face of the prism. This edge is very sharp, the said faces forming an angle of between 48xc2x0 and 50xc2x0 with one another. After some time there will be a large number of chips in this edge. Not only will this reduce the size of the exit face, and thus also the quantity of light emitted, it will also weaken the prism and in addition create a depression that dirt will penetrate into, and where at worst snow clearing equipment can become caught and shatter the whole prism.
According to another aspect of the present invention, the aforementioned problem is solved or least greatly diminished. This is accomplished by rounding the prism edges, as defined in the characterising clause of claim 8 below.
According to a preferred embodiment, the two narrow side faces of the prism are rounded. The object of the present invention is to provide a lighting device of the type mentioned in the introduction, and which remedies or overcomes at least some of the aforementioned disadvantages, and preferably all of them.
A lighting device in accordance with the invention can be made having a practically completely flat outer surface, so as to avoid projections or irregularities that are a nuisance to passing aircraft and other vehicles, and which especially form an obstacle during snow-clearing in the winter season.
A lighting device in accordance with the invention obviates the need for elastic flexibility in the vertical direction. This is due to the fact that the structure, equipped with a supporting means that supports the prism grating, provides considerably greater strength and load capacity. The device will be capable of withstanding repeated, direct stresses from, for example, passing aircraft wheels or snowploughs, without the prism gratings being damaged or loosened. The fact that the device does not have any movable parts means that it has greater long-term operational reliability and durability, under varying and at times extreme environmental conditions.
The structure according to a preferred embodiment, with a light source and associated reflector housed in a casing, fastened to a preferably integrated structure of flange, supporting and fastening means, makes for easier operations in connection with the changing of light sources or other maintenance work which requires access to the internal components of the device. By virtue of this structure, the light source has also been moved closer to the prism grating, which gives better utilization of the light from the light source. Furthermore, the space around the lamp unit and its associated parts is minimized, which reduces problems of condensation associated with temperature gradients and temperature changes.
An inclination corresponding to an angle xcex2 between the main axis of the light source and the plane of reflection from the lighting device, where xcex2 has a value of from about 48xc2x0 to about 50xc2x0, preferably approximately 48xc2x0, optimizes the direction of emergence of the light with a view to ensuring that this direction is as close to horizontal as possible, and also optimizes the light intensity emitted.
The lamp units are preferably housed in a casing in direct, metallic, thermally conducting contact with the supporting means, which in turn is in metallic, thermally conducting contact with a cover. This allows good heat dissipation to a large, integral metal face, which in turn has a contact face against the external surroundings. The improved heat dissipation from the lamp units results in a lower operating temperature and a longer life for the light sources.